linux-kernel - Re: [PATCH v2] ANDROID: binder: change down_write to down

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Date:   Thu, 29 Mar 2018 11:01:51 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Arve Hjønnevåg <arve@...roid.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Todd Kjos <tkjos@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Martijn Coenen <maco@...roid.com>
Subject: Re: [PATCH v2] ANDROID: binder: change down_write to down_read

On Wed, Mar 28, 2018 at 06:59:41PM -0700, Arve Hjønnevåg wrote:
> On Wed, Mar 28, 2018 at 6:00 PM, Minchan Kim <minchan@...nel.org> wrote:
> > binder_update_page_range needs down_write of mmap_sem because
> > vm_insert_page need to change vma->vm_flags to VM_MIXEDMAP unless
> > it is set. However, when I profile binder working, it seems
> > every binder buffers should be mapped in advance by binder_mmap.
> > It means we could set VM_MIXEDMAP in bider_mmap time which is
> > already hold a mmap_sem as down_write so binder_update_page_range
> > doesn't need to hold a mmap_sem as down_write.
> >
> > Android suffers from mmap_sem contention so let's reduce mmap_sem
> > down_write.
> >
> > Cc: Todd Kjos <tkjos@...gle.com>
> > Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Reviewed-by: Martijn Coenen <maco@...roid.com>
> > Signed-off-by: Minchan Kim <minchan@...nel.org>
> > ---
> > From v1:
> >   * remove WARN_ON_ONCE - Greg
> >   * add reviewed-by - Martijn
> >
> > Martijn, I took your LGTM of v1 as Reviewed-by. If you don't like it
> > or want to change it to acked-by, please, tell me.
> >
> >  drivers/android/binder.c       | 2 +-
> >  drivers/android/binder_alloc.c | 6 +++---
> >  2 files changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> > index 764b63a5aade..9a14c6dd60c4 100644
> > --- a/drivers/android/binder.c
> > +++ b/drivers/android/binder.c
> > @@ -4722,7 +4722,7 @@ static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
> >                 failure_string = "bad vm_flags";
> >                 goto err_bad_arg;
> >         }
> > -       vma->vm_flags = (vma->vm_flags | VM_DONTCOPY) & ~VM_MAYWRITE;
> > +       vma->vm_flags |= (VM_DONTCOPY | VM_MIXEDMAP) & ~VM_MAYWRITE;
> 
> Why did you change this to |=? You are no longer clearing VM_MAYWRITE.

Oops, Thanks for pointing out.
I will respin.